Dec.20 (GMM) Daniel Ricciardo's surprising pace was "probably a factor" in the decision taken by Sebastian Vettel to leave Red Bull.

That is the claim of team boss Christian Horner, who admitted it is "weird" to see the quadruple world champion already wearing Ferrari red after so much success with the premier energy drink-owned team.

German Vettel caught everyone by surprise in early October when he announced he is leaving Red Bull.

But it was already known that the 27-year-old, wearing the number 1 on his car for the fourth season in a row, was no fan of the new turbo V6 formula.

"He didn't like what F1 had become and was quite vocal about it -- didn't like the engines, the noise, the way the car felt," Horner told the British broadcaster BBC's season review.

The Briton said it was "Seb and Kimi (Raikkonen)" - now to be Ferrari's race pairing for 2015 - that suffered the most with the radical new regulations.

"He (Vettel) got very frustrated the car wasn't doing what he wanted it to do and of course to compound that his teammate is winning a couple of races and performing at the level he was."

Vettel's teammate, of course, was Australian Ricciardo, stepping up from Toro Rosso and surprising F1, Red Bull and even himself with his stellar form.

Ultimately, Ricciardo won three grands prix compared to Vettel's none in 2014, and finished the drivers' title behind only the dominant Mercedes duo.

Horner thinks Ricciardo's shock form was "probably a factor" in Vettel's decision to move, with a "defining moment" coming at Monza where the German was audaciously overtaken by his teammate.

"He was enormously frustrated after that grand prix," said Horner.

"It was at a time where, knowing Sebastian as well as I do, I could see he was very distracted and it was obvious something was at the back of his mind," he added, referring to the advanced negotiations with Ferrari.

"By the time he got to Singapore you could see he was a different person," Horner said. "So, no, it wasn't a great surprise in the end."